1. Field of the Invention
The present invention relates to an exhaust emission purifying apparatus adapted for Use in purifying exhaust emission from a diesel engine, a gasoline engine or the like, mounted on an on-moving vehicle by supplying a reducing agent to the flow of exhaust emission on an upstream side of a reduction catalytic converter in an exhaust passage of the engine, to thereby reductively eliminate nitrogen oxides (NOx) emitted from the engine. More particularly, it relates to an exhaust emission purifying apparatus for an engine capable of removing a component of a reducing agent, which is deposited in an exhaust passage as a result that the supplied reducing agent is attached to or deposited on the inside of the exhaust passage
2. Description of the Related Art
Several types of exhaust emission purifying apparatuses has been proposed respectively formed as an on-moving vehicle emission cleaning system for cleaning the exhaust emission by eliminating especially NOx in harmful substances in the exhaust emission exhausted from an engine on the vehicle. In this exhaust emission purifying apparatus, a reduction catalytic converter is disposed in an exhaust passage of the engine, and a reducing agent is supplied by injection into the exhaust passage on an upstream side of the reduction catalytic converter, so that NOx in the exhaust emission and the reducing agent are subjected to the catalytic-reduction reaction to each other, thereby purifying NOx by conversion into a harmless component. The reducing agent is stored in a storage tank in a liquid state at the room temperature, and a necessary amount thereof is injection-supplied from an injection nozzle. For the reduction reaction, ammonia having good reactivity to NOx is used, and as the reducing agent, the urea aqueous solution, which is hydrolyzed to readily generate ammonia, the ammonia aqueous solution or other reducing agent aqueous solution, is used (refer to Japanese Unexamined Patent Publication No. 2000-27627, indicated below).
In the above-described conventional exhaust emission purifying apparatus, an amount of the reducing agent, for example the urea aqueous solution (to be referred to as “urea water”), to be injection-supplied to the exhaust passage, is controlled according to engine operating conditions, such as an exhausting amount of NOx, the exhaust emission temperature and the like. However, depending on the exhaust emission temperature, urea in the supplied urea water may be deposited at the periphery of the injection nozzle, in a muffler diffusion chamber and the like, in the exhaust passage. In such a case, there is a possibility that the exhaust emission pressure of the engine rises resulting in a degradation of engine power or in a degradation of fuel consumption. Further, also in the case where light oil or gasoline being a fuel, or alcohol, is used as the reducing agent, similarly to the above, a component such as carbon or the like may be deposited in the exhaust passage, resulting in a possibility of the degradation of engine power or the degradation of fuel consumption.
Accordingly, in the case where the reducing agent component is deposited in the exhaust passage as described in the above, it is necessary to remove the reducing agent component. However, in the conventional technology, it has been difficult to readily accomplish the removal.